Eating a handful of nuts every day, along with the right diet, could help fight heart disease, a new study shows.

Spanish researchers have found that walnuts, hazelnuts, and almonds were all helpful in reducing belly fat and improving cholesterol and blood pressure.

The study, which will appear Monday in the Archives of Internal Medicine, also revealed that nuts combined with a Mediterranean diet rich in fruit, vegetables, and fish, was more effective than a low fat diet.

"What's most surprising is they found substantial metabolic benefits in the absence of calorie reduction or weight loss," said Dr. JoAnn Manson, chief of preventive medicine at Harvard's Brigham and Women's Hospital.

Manson did not participate in the study, but warned that with an American diet, it's best not to add nuts unless another snack is taken away.

"Using nuts to replace a snack of chips or crackers is a very favorable change to make in your diet," she said.

Dr. Jordi Salas-Salvado, lead author of the study, explained that nuts help people feel full while burning fat at the same time.

"Nuts are rich in anti-inflammatory substances, such as fiber, and antioxidants, such as vitamin E," he said. "They are high in unsaturated fat, a healthier fat known to lower blood triglycerides and increase good cholesterol."

More than 1,200 Spaniards ages 55 to 80 participated in the study. Those who were told to eat three whole walnuts, seven or eight whole hazelnuts and seven or eight whole almonds showed the most improvement.

None of the participants had a history of heart disease, but some had risk factors like diabetes, high blood pressure, and obesity.